that's a poor excuse to not wear a helmet. I guess if you are in a city and the only thing that you worry about are cars then it's reasonable.

Where I am, there are more things than cars that will take me out, and a helmet is one thing that protects me.

Low hanging or falling branches, massive potholes that can kill your tires and bike wheel, people carrying large objects out of the trunk of their car - so even if they are driving safely, it only takes one snapped bungie cord to have a piece of furniture come at you...

Always wear a helmet regardless of how safe you think cars are being!